Technology update

Aug 30, 2012

Nanocrystals go ferroelectric

What is the smallest scale on which ferroelectricity can exist? Down to just a few nanometres say a team of researchers in the US who have been studying maps of ferroelectric distortions in germanium telluride and barium titanium oxide. The result could be important for making next-generation non-volatile memory devices that would store terabits of data per square inch, says the team.

Ferroelectric materials have a permanent dipole moment, like their ferromagnetic counterparts. However, in ferroelectrics, the dipole moment is electric and not magnetic and so can be oriented using electric fields rather than magnetic ones. This opens up a host of novel device applications because it allows electrically digital information to be stored in ferroelectric thin films. Indeed, scientists have been eyeing up ferroelectrics as candidate materials for the next generation of non-volatile memory devices for quite a while now.

In their work, researchers led by Paul Alivisatos and Ramamoorthy Ramesh at the Lawrence Berkeley National Laboratory and the University of California at Berkeley sought to answer a longstanding question in the field of ferroelectrics. This question relates to the ultimate size limit of stable ferroelectric ordering and the form that any such ordering would assume at the smallest length scales, explains team leader Mark Polking of UCB.

“Previous research in the field had suggested the complete disappearance or disordering of the atomic displacements that give rise to ferroelectricity, or the emergence of a vortex-like arrangement of these distortions,” he told nanotechweb.org. “The results from our study indicate that local atomic displacements remain largely linearly ordered in a single domain, leading to a net electrical polarization.”

This means that useful ferroelectric properties, including polarization switching and piezoelectricity, can be maintained down to dimensions of just a few nanometres.

Imaging ferroelectric polarization

The researchers analysed ferroelectric ordering in single nanocrystals of GeTe and BaTiO3. GeTe is a semiconducting ferroelectric while BaTiO3 is a typical oxide ferroelectric. They began by directly imaging the structural distortions associated with ferroelectricity using aberration-corrected transmission electron microscopy of individual particles. Next, they characterized the correlations of the distortions further using atomic pair distribution function analysis of particle ensembles. “We then imaged the ferroelectric polarization directly using electron holography and measured piezoelectric hysteresis loops of the singe nanocrystals,” explained Polking.

“Our results indicate that non-volatile memories made from these ferroelectric nanocrystals could have data-storage densities in the Tbit/in2 regime,” he said. “One could also imagine using such materials as nanoscale piezoelectric actuators and transducers in future nanoelectromechanical systems (NEMS) devices, among other potential applications.”

The researchers, who reported their work in Nature Materials, say that they would now like to find out whether vortex polarization states predicted by theory could be stabilized in these particles. “For example, heterostructures composed of a ferroelectric core and an epitaxial insulating shell could allow for the first observations of true monodomain vortex states in a ferroelectric material,” said Polking.